1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus and image forming method by which an image can be fixed satisfactorily to a recording medium and the strength of a formed image can be improved.
2. Description of the Related Art
An inkjet recording apparatus forms an image on a recording medium by successively depositing droplets of ink onto the recording medium. The inkjet recording apparatus is able to record images of good quality by means of a simple composition, and therefore such apparatuses are widely used as domestic printers for individual use and office printers for commercial use. In the case of office printers for commercial use, in particular, there are increasing demands for higher processing speed and higher image quality.
An inkjet recording apparatus forms an image on a recording medium using ink and fixes the image onto the recording medium by drying and then melts resin particles contained in the ink so that the ink forms a film. A known fixing device for fixing an image on a recording medium is an apparatus which fixes a formed image on a recording medium by the action of heat and pressure, by holding and conveying a recording medium on which an image has been formed using a heater, a fixing roller, a conveyance drum, and the like.
However, if the heating temperature during fixing is high, then an offset effect occurs in which a portion of the ink is transferred to (adheres to) the fixing roller. This offset effect causes problems in that, when recording in a continuous fashion, the ink is transferred again onto the non-image portion (non-recording portion) of the next recording medium and therefore a satisfactory printed item cannot be obtained.
In order to solve problems of offset such as this, the minimum filming temperature (also abbreviated as “MFT” below) of the resin particles contained in the ink is an important factor. By carrying out fixing at a temperature equal to or higher than this minimum filming temperature, the resin particles are melted and image fixing can be performed.
As an apparatus or method which forms images by specifying the minimum filming temperature, Japanese Patent Application Publication No. 3-160068 discloses, for example, an inkjet or ink mist recording method which uses an ink having a minimum filming temperature of 40° C. or higher. Furthermore, Japanese Patent Application Publication No. 8-283636 discloses a recording method comprising at least a first step of fixing a printed item by means of a platen heated to at least the MFT of a low-MFT resin emulsion having an MFT of 60° C. to 100° C., using ink containing the low-MFT resin emulsion and a high-Tg resin emulsion having a Tg (glass transition temperature) of 140° C. to 200° C., and a second step of carrying out repeat fixing of the printed item by further heating outside the recording apparatus.
However, the ink described in Japanese Patent Application Publication No. 3-160068 specifies the MFT temperature in order to improve nozzle blockages and storage stability, and provides no investigation with respect to fixing conditions. Furthermore, the recording method described in Japanese Patent Application Publication No. 8-283636 discloses a recording method in which fixing is carried out by heating at least to the MFT of the resin emulsion contained in the ink, but this MFT is the MFT when dispersed in water and no investigation is made in respect of the MFT and fixing conditions in a solvent composition which is close to a dry state. Furthermore, since an image is fixed by heating a recording medium from the rear surface, there have been no problems of the occurrence of offset due to excessively high heating temperature during fixing.
Conversely, if the MFT temperature is low, then the resin emulsion is molten at room temperature, and when recording media on which images have been formed are stacked upon each other, the problems of image peeling and transfer of color to other recording media (blocking) also arise and it has been difficult to obtain images of high quality.